1. Field of the Invention
The present invention relates to an image signal processing apparatus for processing an input image signal in accordance with the characteristics of the signal.
2. Description of the Prior Art
In order to provide gradation or gray levels in a highlight image portion, a halftone image portion and a shadow image portion in a printer or a scanner, it is known to perform input/output conversion in accordance with the gamma-correction curves a, b and c, in FIG. 1, that is, gamma-correction.
In order to provide gray levels for various types of input images, it is essential to obtain an optimal gamma-correction curve suitable for each image. This has required trial and experience.
In a printer producing a hard copy of a color image signal, there is a limit in the density range which allows image reproduction. For this reason, it is preferable to set an upper extremity (upper limit) and/or lower extremity (lower limit) for an input level of an input image signal and to normalize the signal accordingly, and then to process the image signal. However, an input image signal is very rarely distributed uniformly along the entire density range. In general, the brightness distribution of an input image signal is generally concentrated at a dark or bright side. When normalization is performed with a fixed upper extremity and/or a fixed lower extremity, an output image has a poor gray level quality deviating either to the bright or dark side. In view of this, it is preferable to adjust the upper and/or lower extremity in accordance with each input signal. However, when a manual adjusting method using an adjustment volume or the like is used, the output quality depends upon the experience and skill of the operator, and optimal image quality cannot be guaranteed at all times.